Disable "Bootstrap" test by default (it requires network).

Index: src/bootstrap/src/core/build_steps/test.rs
--- src/bootstrap/src/core/build_steps/test.rs.orig
+++ src/bootstrap/src/core/build_steps/test.rs
@@ -50,7 +50,7 @@ pub struct CrateBootstrap {
 impl Step for CrateBootstrap {
     type Output = ();
     const IS_HOST: bool = true;
-    const DEFAULT: bool = true;
+    const DEFAULT: bool = false;
 
     fn should_run(run: ShouldRun<'_>) -> ShouldRun<'_> {
         // This step is responsible for several different tool paths.
